This post is part of a series of quick astrophysics explainers I’m trying to put together, partly so that I can link to something when I talk about some of these things elsewhere rather than frequently repeating myself! Hopefully it shouldn’t take more than a few minutes to read, and you’ll come away knowing a little more about gravitational waves.

I frequently talk about gravitational waves, and new discoveries which are being made by observing them. But what is a gravitational wave?

To understand what a gravitational wave is it’s probably easiest to think first about something a bit more familiar. Light. Using light to make observations of astronomical objects and phenomena is probably what your mind goes to when you think about astronomy. Think of the Hubble Space Telescope, which uses normal light to make some of the most important observations in astrophysics over the last thirty years.

Light, and other forms of electromagnetic radiation, are generated by the electrons inside objects accelerating and decelerating. In astronomy a common source of this acceleration comes from electrons moving between “energy levels”, and observing the light which comes from this allows us to work out the chemical composition of what we’re looking at.

When it comes to gravitational waves things are similar. Light and electromagnetic radiation are produced by electrons, and it’s the fundamental charge of those electrons which is responsible for the production of light. Charge is the quantity matter can have which allows it to interact electromagnetically. For gravity the equivalent quantity is mass; the size of the masses involved in a gravitational interaction affects the strength of the interaction. It follows then, that gravitational waves are generated by masses accelerating and decelerating.

Gravity is a little different from other forces in nature, or at least, the way that we understand it is different. Gravity operates by altering the geometry of the universe itself; masses cause the structure of the universe to bend around them. When masses move the structural changes in that geometry change to follow the masses. When the masses are accelerating those changes produce a ripple effect throughout the geometry of the universe, and travels through the universe at the speed of light. This is a gravitational wave.

Any accelerating mass will produce a gravitational wave, but it turns out that the amount of curvature a mass produces in spacetime, that underlying geometry of the universe, is absolutely tiny. You need something very heavy to make a wave which can be observed a long way off. This basically restricts us to making observations of the heaviest, densest things in the universe: black holes, and neutron stars (though there are a few other potential sources which, for the moment, I’m skipping over; maybe one for a future blog).

In comparison to watching-out for the acceleration of electrons, watching out for the acceleration of things like black holes means that we can be waiting a while before we see anything, and the number of situations which will actually produce a gravitational wave are a little more limited. Fortunately, some fairly normal processes will produce an acceleration, which produces a gravitational wave, such as two black holes orbiting each other. These gravitational waves, however, have a very low frequency, which makes them difficult for the current generation of detectors to observe. Producing gravitational waves does make the orbits lose energy, however, which ultimately makes the orbits shrink, and eventually binary black holes will produce gravitational waves at a high enough frequency that we can observe them. Right before they crash into one another. Of course, we can observe the gravitational waves produced by that crash too, which are the biggest explosions to have occurred in the universe since the Big Bang.

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